As an improvement over the spray painting of articles such as automobile bodies, boats and household appliances, a new kind of paint-coated sheet material or film has been developed to provide protective and decorative finishes. The new material comprises a flexible, stretchable, thermoplastic support sheet, also known as a carrier film, which has a protective and decorative paint layer, also called a basecoat, of uniform thickness and appearance on one side and, optionally, an adhesive layer on the other side. It can also have other layers such as a tie or bonding layer between the paint layer and the carrier film. In a preferred embodiment a transparent topcoat, also called a clearcoat, covers the paint layer. This combination provides an attractive basecoat-clearcoat or clear-over-color appearance which in recent years has become popular for spray-coated finishes.
Using known thermoforming procedures such as vacuum forming and in-mold bonding, preferred types of the paint-coated sheet material can be stretched and bonded to an article such as an automobile body panel to provide a basecoat-clearcoat finish. Advantages over obtaining such finishes by spray painting include economy in the use of paint and improved control of evaporating solvents, which reduces air pollution. Furthermore, the new material has a remarkably smoother and more attractive appearance than spray painted finishes.
The new type of thermoformable sheet material and a process for its manufacture are described in the G. G. Reafler, U.S. patent application Ser. No. 116,426, filed Nov. 3, 1987. The process involves applying the paint composition to the surface of the thermoplastic carrier film by laminar flow coating, followed by drying and then coating and drying each additional layer in sequence to obtain a paint-coated film of excellent gloss and smoothness.
In the Reafler sheet materials, the carrier film has heat-softening and tensile elongation properties which adapt it to use in thermoforming. The various layers have compatible heat-softening and tensile elongation properties. As a result, the sheet material can undergo substantial elongation during thermoforming without crazing or delamination of the layers.
In use, the paint-coated film is stretched and bonded by thermoforming to a substrate such as a three-dimensional automobile panel. The procedure of thermoforming can be substantially as described in the patent to Short et al, U.S. Pat. No. 4,769,100 wherein the film is heated and then drawn by vacuum into bonding contact with the substrate.
Notwithstanding its excellent thermoforming capability, areas of this high quality sheet material can become non-uniform or flawed by the introduction of visible defects during manufacture or storage of the film.
As in the large-scale manufacture of any product required to meet stringent quality standards, it is inevitable that during the production of a paint-coated film which is expected to provide a consistently uniform finish on the exterior of an automobile and the like, defective portions of production runs have to be discarded as a quality control measure.
It is of course time consuming as well as economically wasteful to detect and discard such flawed portions. By flawed portions we mean sections of film which have coating patterns, streaks, blemishes or other visible defects formed during the coating operation or that may have developed during storage or handling of the finished film.
And more so than in the manufacture of materials that remain physically unchanged during their intended use, in paint-coated films which are heated and stretched during their bonding to intricate three-dimesional parts, any flow tends to become magnified in proportion to the degree of stretch and is therefore seriously objectionable.
The present invention provides a surprisingly simple method for curing such flaws and thereby obviates the need for the time consuming and economically wasteful detection and discarding of defective portions of the film. No new equipment is required. Nor is it necessary to modify or expand existing equipment.